OK, so this isn't really the type of thing you'd call Nathan Drake in for but I only discovered it by accident. My laptop just happened to be on a desk next to my big carry case of Dreamcast games the other day so I decided to mindlessly pop a few of the GDs into the optical drive to see if there was anything interesting in the way of bonus content on them.
I was aware that some games (such as Shenmue and Shenmue 2) do have folders of images and other bonuses on them, but I didn't know about Fur Fighters. If you're unfamiliar with the name, Fur Fighters is a third person shooter from Bizarre Creations and is a rather excellent little game...and if you put the disc in your computer, you'll find a folder on there that's full of exclusive artwork.
The art is mainly concept designs for levels and characters, and it all looks quite lovely. Not a massive story, I know - but a cool find nonetheless and it makes me want to go through my entire collection looking for more hidden bonuses left behind for the inquisitive gamer.